Bonding of rubber vulcanizates to substrates, especially metal is conventionally obtained by two-coat primer-overcoat adhesive systems or one-coat primerless systems. In order to provide acceptable bonding, adhesive compositions must exhibit excellent bonding as retention of rubber on the substrate after bond destruction, adequate sweep resistance i.e., ability of the uncured adhesive coating on the substrate to remain undisturbed against the force of injected green rubber into the mold cavity, good storage stability of the wet adhesive and durable adhesion under extreme environmental conditions, typically measured by the hot tear test (ASTM D-429) boiling water and salt spray tests (ASTM B-117-97, for example).
The treatment of metals using zinc phosphate in conversion coating processes is well known, for example as disclosed in U.S. Pat. No. 6,019,858.
In the literature relating to adhesives for bonding rubber to metal (RTM), the following additives such as organosilanes, dispersing agents, adhesion promoting resins such as phenol formaldehyde, crosslinkers such as nitrosobenzenes, and maleimide compounds, carbon black, silica, calcium carbonate, oxides of the metals Al, Ca, Zn, Mg, Pb, Zr, also zirconium salts, e.g. zirconium aluminate, and lead salts of inorganic and/or organic acids, e.g. basic lead carbonate. The use of lead compounds is widely practiced in RTM adhesives because these materials impart essential heat and corrosion resistance of the bond between the vulcanized elastomer and the metal.
U.S. Pat. No. 3,640,941 describes a one-coat rubber-to-metal adhesive containing four essential ingredients: (a) a graft polymer of a polybutadiene and a substituted cyclopentadiene monomer, (b) dibasic lead phosphite, (c) resorcinol and (d) a volatile solvent. In this adhesive system between 25-150 parts by weight of dibasic lead phosphate per 100 parts of polymer is described as necessary to achieve the desired performance.
U.S. Pat. No. 4,119,587 discloses a one-coat adhesive composition comprised of the three essential constituents: (a) halogenated polyolefinic, (b) aromatic nitroso compound, and (c) lead salts.
U.S. Pat. No 5,268,404 discloses adhesives comprising a halogenated polyolefin, an aromatic nitroso compound, metal oxide such as zinc oxide or magnesium oxide, and optionally a vulcanizing agent such as sulfur or selenium, a phenolic epoxy resin, or carbon black.
Lead compounds useful as additive in RTM adhesives provide either an acid scavenging feature and/or corrosion resistance in conjunction with halogenated polymers. Due to the increasing demand from both government and industry to use adhesive materials that do not contain bio-accumulative ingredients. Conventional rubber-to-metal adhesives have required effective amounts of lead compounds and selenium to provide essential resistance to heat and corrosion. It would be desirable to provide adhesives for bonding of rubber to metal during the vulcanization processes that contain less than 1000 ppm of undesirable ingredients such as lead and selenium-containing compounds while at the same time providing comparable heat and corrosion resistance.
The elimination of additives based on lead down to ppm levels presents a problem of finding a suitable replacement that provides for versatile performance in respect to the variety of vulcanizable elastomer types. In addition to the problem of versatility, especially in respect to one-coat adhesive systems, is a general inability to afford optimum adhesion, particularly at elevated temperatures, poor storage stability, poor resistance to pre-bake, poor corrosion resistance and poor resistance of the adhesive bond to environmental conditions such as solvents, moisture and the like.
In view of the potential adverse environmental effects posed by the presence of lead, and in light of the technical challenge for minimizing lead levels, it would be highly desirable if a one-coat rubber-to-metal bonding adhesive composition could be developed which possesses all the aforementioned characteristics without the need of lead compounds.